Brushless direct current (dc) motors are used in a variety of applications, such as to rotate a data storage medium in a data storage device. Brushless dc motors, also referred to as spindle motors, include one or more phase windings and a corresponding array of permanent magnets. Current is selectively applied to the phase windings to establish magnetic fields which interact with the magnets to induce rotation of the motor.
A series of commutation states are defined for different mechanical alignments of the phase windings and the magnets. The motor is commutated by selectively switching the particular phase windings to which current is sourced and from which current is sunk as the motor rotates through these commutation states.
Back electromotive force (bemf) commutation can be employed to control the motor at higher operational velocities. As those skilled in the art will recognize, bemf commutation relies on sensing the bemf generated in the phase windings as a result of motor rotation. Commutation states are changed in relation to zero crossing points of the voltage on non-energized phase windings.
At lower intermediate motor velocities, the interaction between the phase windings and the magnets may be insufficient to permit bemf control to accelerate and maintain rotation of the motor. Accordingly, various approaches have been proposed in the art to accelerate a spindle motor from rest to an intermediate velocity, after which a hand-off takes place to transfer operational control to bemf circuitry which continues the acceleration of the motor to the final operational velocity.
While operable, there remains a continued need for improvements in the art whereby a spindle motor can be effectively controlled in a closed-loop fashion, such as but not limited to acceleration to an intermediate velocity for bemf commutation hand-off.